Article excerpt

There's dry and then there's dry.

Although researchers have known that the atmosphere of Mars contains very little water vapor, a newly reported study shows that in December 1990 the Martian atmosphere contained the smallest concentration of water vapor ever recorded for the Red Planet. Indeed, if all the vapor then present in the atmosphere had condensed on the planet's surface, it would have formed an ocean only 3 micrometers deep -- too shallow to cover even the thickness of a human hair.

The study marks the first time researchers have used a ground-based instrument -- in this case, the Very Large Array radiotelescope near Socorro, N.M. -- to measure the thermal radio emissions of water in a planetary atmosphere other than Earth's. Previous surveys, both in space and on the ground, have relied on an entirely different technique to measure the concentration of Martian water vapor.

Past surveys, notes study coauthor R. Todd Clancy of the University of Colorado at Boulder, employed near-infrared detectors that record how much sunlight the vapor absorbs. In particular, infrared instruments aboard the Viking spacecraft in the mid-1970s found twice as much water vapor during the same Martian season, early northern spring, as the 1990 study; a 1988 infrared study from the ground revealed four times as much water vapor. Clancy and his colleagues, Arie W. Grossman of the University of Maryland in College Park and Duane O. Muhleman of the California Institute of Technology in Pasadena, report their work in the November Icarus.

Bruce M. Jakosky, also at the University of Colorado, cautions that scientists have not yet rigorously compared the infrared absorption and radio-emission methods. But if the apparent variation in water vapor proves accurate, he says, it suggests that the concentration of water in the atmosphere varies as much from year to year on Mars as it does from season to season.

Water may have played a key role in etching the rugged face of the Red Planet, and it remains an influence on climate. …

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